1. Field of the Invention
The invention relates to the construction of a microlensed optical fiber terminals for various parts, such as optical switches, optical merging/branching filters, optical isolators, and optical connectors, including polarization-independent optical isolators for optical amplification, as well as to such optical systems utilizing microlensed optical fiber terminals and methods for producing such terminals and optical systems. Specifically, the invention also relates to microlensed optical isolators and methods for their manufacture, especially methods of coupling optical fibers and optical couplers for performing such coupling.
2. Description of Related Art
In the development of optical communications, miniaturization of the optical devices and parts used therein has become a desired objective. In particular, optical isolators, optical circulators, and optical merging/branching filters are in demand which achieve both miniaturization and simplification when they are coupled with optical fibers. Additionally, in recent years, a distributed-feedback laser with a narrow spectrum has come into use for high speed optical communications systems which is sensitive to back reflection. Thus, it has become necessary for the terminals of optical fibers to have high return loss characteristics.
In general, in the case of pigtailed optical isolators with optical fibers, when they are coupled as shown in FIG. 2, the light emitted from optical fiber 1 enters into the optical device 4 as parallel beams from either the spherical lens 2 or the refractive index-distributed lens 3, and the exiting light is recollected into the coupled optical fiber 1 in the same manner.
Conventional collimating systems have problems adjusting the optical fiber and the optical axis position of the lens, and its fabrication is expensive, resulting in a high cost for optical fiber collimating products. Furthermore, as shown in FIG. 3, in the case of a conventional system, reflection has been prevented by using an organic substance as a refractive index matching agent 5. Consequently, there are anti-weather and anti-thermal issues. In addition, an anti-reflective film is formed onto the optical entrance and exit surface 6 shown in FIG. 3 while the optical fiber cord is attached, so that a hard coat, which requires heating to about 300.degree. C. in the case of ordinary vapor deposition, is not used due to the heat sensitivity of the optical fiber sheath and the gas generated. Instead, ion-assist coatings are employed; but, use these coatings is disadvantageous in that they are expensive and uniformity is difficult to obtain.
Furthermore, from the standpoint of miniaturization, fiber collimator beams of sufficiently narrow (e.g. less than 200 .mu.m) light flux are required. However, conventional optical fiber collimators are not able to obtain a light flux narrower than 300 .mu.m, and a return loss of only approximately -27 dB was obtainable. Thus, it was necessary to use a complicated coupling configuration, such as coupling the collimator with the lens system by forming an angle at the tip of the fiber.
Still further, optical communication and transmission systems require optical amplification to compensate for optical attenuation. In recent years, electrical amplification (i.e., optical signals are converted to electrical signals which are electrically amplified and then reconverted back to optical signals) has been replacing the complicated and readily noise-overlaid conventional method of amplifying optical signals by photoelectric conversion. Techniques for optical amplification of optical signals have also been developed, such as semiconductor amplification and rare earth doped optical fiber amplification has been developed, and rapid progress in optical as well as optical fiber transmission is anticipated for information transmission media.
With regard to optical amplification, although a large amplification factor for the signal beam is a fundamental issue, it is also important that noisy signals should not be amplified. Consequently, it is believed to be essential that every optical amplification relay be equipped with an optical isolator and that the reflective return beam be cut off. Moreover, since a signal beam propagates through an optical fiber, the polarization surface fluctuates randomly. Therefore, a polarization-independent optical isolator is necessary, and at the same time, it is essential that the polarization-independent optical isolator has optical fiber terminals (pigtails) attached at both ends, so that it can be installed between optical fibers.
In response to market demand, several configurations have been proposed for a polarization-independent optical isolator with pigtails, and representative examples thereof are shown in FIGS. 4-6. More specifically, FIG. 4 shows a method using wedge-form birefringent plates 11 where, in the case of a pigtailed optical isolator, having a Faraday rotator 15 and optical fibers 1 at both ends, generally, optical coupling is performed by having the beam emitted from one optical fiber 1 enter the optical device as a parallel beam by means of a spherical lens 2, or a refractive index-distributed lens, and converge into the other optical fiber 1 in the same manner after exiting.
In optical coupling systems of the type described relative to FIG. 2, in the preceding paragraph, a problem exists in adjusting the optical fibers and the optical axis position of the lens, which must be performed at the submicron level, and is costly in assembling equipment, etc. That is, the two optical fibers and two lenses are independent parts which must be mutually aligned, resulting in an expensive collimating system, and optical coupling efficiency greatly decreases as a consequence of shifts in the optic axis and the angles between the optical fiber and the lens. However, for optical systems including optical fiber collimator products and optical fiber coupling systems, they are relatively simple as compared to configurations as shown in FIGS. 5 & 6.
The configuration of FIG. 5 has advantages, such as the use of a parallel birefringent plate 14, whereby the special processes shown in FIG. 4 are unnecessary. By arranging the magnetic orientation of the Faraday rotator 15 in the reverse direction, the temperature characteristics inherent to a Faraday rotator can be mutually compensated. However, it is a complicated configuration with a large number of components.
The configuration of FIG. 6 is an intermediate construction. There are no small high performance materials serving as active substances for the optically active plate 16. When the most suitable crystalline plate is used, a thickness of approximately 11 mm in the 1310 nm band and approximately 15 mm in the 1550 nm band are required. A half-wave plate may also be used instead of the optically active substance; it is still functional after it gets worn. However, the shortcoming exists that angular fluctuations with respect to the optical axes are severe due to the development of elliptical components caused by changes in plate thickness. In addition, since optically parallel shifts are used as the isolation method in both FIGS. 5 & 6, it is necessary to sufficiently narrow the light flux of the Gaussian beam propagating between the optical fibers. Although, at less than 60 .mu.m, an interlens distance of less than 5 mm is obtainable at most. As a result, building of an optical system becomes a problem.
The optical isolator configurations described above have both merits and demerits; it is difficult to judge their technical and economic advantages and disadvantages. Nonetheless, when coupled with optical fibers, the return loss from the ends of the optical fibers and lens surfaces must be controlled in all of these configurations so as to be kept at or above the optical isolator characteristics. Preferably, a return loss of above -50 dB is necessary. The most universal and accurate near-end reflected beam control methods are those of FIGS. 4-6, in which a configuration is proposed for the adhesion of optical fibers 1 to a one-side gradient glass 17 having a refractive index equal to refractive index matching resin. A high return loss of approximately -60 dB is assured; but, the configuration is complicated. The shortcoming of an increased number of components has not been avoided, and because adhesion involves the use of organic substances on the one-side gradient glass, shortcomings in terms of weather and temperature resistance exist. In other words, a polarization-independent optical isolator with pigtails requires detailed designs not only with respect to isolator construction, but also in implementation of optical coupling between the pigtails. To date, there has been no economical method developed which overcomes these technical drawbacks and which can be advantageously used for mass production.
In recent years, attempts have been made to form microcollimator beams. In the Journal of Lightwave Technology, Vol. LT-5, No. 9(1987), William L. Emkey et al. propose coupling microcollimator beams as narrow as 40 .mu.m by fusing a multimode refractive index-distributed fiber (called "MMGIF" hereafter) to a single mode fiber (called "SMF" hereafter), and they reported an optical coupling system at a distance of up to about 3 mm is obtainable at a coupling loss of 0.1-1.6 dB. The method of its manufacture involves an arc-discharge fusion of SMF 18 to MMGIF 19 as shown in FIG. 7(a), and cutting of the MMGIF 19 to a desired length, as shown in FIG. 7(c), by scratching with a scribing tool 20 as in FIG. 7(b). In this case, the lens 21, which has a convergent function and a system for controlling the convergent pitch length, is formed by the MMGIF itself.
However, in the configuration using a MMGIF and a SMF, there are shortcomings such as the fact that a light flux of more than the MMGIF core diameter is theoretically impossible, so that a greater value than 50-62.5 .mu.m is impossible. Also, because of the sharp decrease in the coupling loss at a distance of over 3 mm, consequently, there is no degree of freedom in the collimating distance. Furthermore, the distribution of the refractive index in the MMGIF fiber segment and adjustment of the wavelength pitch in the manufacturing process must be determined individually, which is unsuitable for mass production and is also expensive.
With regard to this problem, Kevin J. Warbrick proposed an SMF and a non-doped silica fiber lens optical system in Japanese published patent application S 61-264304 as an attempt to solve this problem. However, the curvature of the lens segment is restricted to a lens radius of 62.5 .mu.m due to diffraction loss. Thus, the beam obtained is approximately 60 .mu.m, and structurally, about 80% is, at most, the limit for the silica fiber diameter, which is too narrow for insertion into an optical device. In other words, a beam of about 60 .mu.m is, conversely, too narrow and unsuitable for interlens coupling 82 by inserting a polarization-independent optical isolator into an optical device, i.e., for light coupling with a Gaussian Beam. Therefore, the real problem is to how to produce a 60-200 .mu.m beam.
In Japanese published patent application H1-126609, a manufacturing process is disclosed in which the SMF tip is heated in an acr-discharge to form a spherical lens at an optical fiber tip. When this method is used, a beam converging system with a longer coupling distance than in the previously described case of the SMF and MMGIF lens proposed by W. E. Emkey can be expected. The main characteristics of this method of forming a lens sphere are as shown in FIG. 8, and involve locating an arc heat source above an optical fiber. The optical fiber is pushed up as much as necessary into a guide hole through a narrow hole of approximately the outer diameter of the optical fiber and fused in the arc discharge unit positioned directly above it.
However, since the fusion unit is above the fiber, the radius of curvature of the spherical surface is enlarged on the beam axis due to the effects of gravity, and the return loss has the potential to be relatively high. Additionally, when a relatively long interlens space of greater than 5 mm is desired, the spherical lens segment must be enlarged and the radius of curvature increased. Therefore, it is difficult to manufacture a highly symmetric sphere.
The present inventors proposed, in Japanese patent application No. H3-17022, an optical fiber terminal for optical coupling as a means for solving the above-mentioned shortcomings and which is essentially composed of an SMF, a non doped silica fiber beam expansion segment, and a non-doped silica spherical lens. As represented in FIG. 9, a first optical fiber is joined to a second optical fiber having the same outer diameter with a refractive index equivalent to the first fiber core. However, in actual mass-production, due to fluctuations in manufacturing parts, relaxing the precision of the beam angle shift .theta. was more important than shifts in the direction of the axes X and Y (see FIG. 1(a) for the axes directions and FIG. 10 which shows that slight shifts in aligning angles .theta. lead to parabolic increases in optical coupling losses to an extent that an angle shift of less than 0.1" must be achieved to obtain a coupling loss of less than the currently required value of 0.2 dB). In addition, the return loss generated from the lens tip had to be controlled to a very small value depending on the field of application. For example, in the case of a polarization-independent optical isolator, greater than 60 dB was necessary, and the return loss of about 40-50 dB was inadequate.